1. Field of the Invention
The present invention relates to selective production of C6-C12 hydrocarbons useful as automotive fuel components. In particular, the present invention concerns a process for treating a fresh olefinic hydrocarbon feedstock in a reactor assembly comprising at least two reaction zones arranged in a cascade for converting lower olefins of the olefinic feed-stock into gasoline grade dimerized components.
2. Description of Related Art
In oil refining processes, several streams containing light olefins emanate from various sources. Light n-olefins can be converted to more valuable hydrocarbon-based gasoline components or to a feedstock for gasoline components by means of several processes, namely, isomerization, etherification, dimerization and alkylation. Thus, in many modern refineries, streams containing these components are treated so that first the C4-C7 isoolefins (e.g. isobutene, 1-methyl-1-butene, 2-methyl-2-butene, 1-methyl-1-pentene, 2-methyl-2-pentene and 2-ethyl-1-butene) are converted to ethers and, then, the remaining raffinate stream is conducted to an alkylation unit, where the remaining—mainly linear—olefins are reacted with isoparaffins. These streams are utilized, for example, in the production of gasoline for automotive engines.
A reaction step where isoolefins are removed before alkylation has a positive effect on the overall operation, because the tertiary ethers resulting from the reactions of the isoolefins are excellent gasoline components. Moreover, isoolefins tend to be too reactive in alkylation and they therefore react readily to yield unwanted side products. Finally, the total amount of olefins present in the feed stream tends to be higher than the amount of available isoparaffins and any alkylate production is limited by the amount of isoparaffins in the feed. When isoolefins are consumed before alkylation, the total amount of gasoline components increases and the quality of the combined product is improved.
This processing chain can be improved, if the feed is totally or partially treated in a skeletal isomerization unit before the etherification. Then, the yield of ethers can be increased because linear olefins are converted to isoolefins and the ratio of isoparaffins to olefins in the remaining raffinate stream is closer to optimal because a larger proportion of the olefins reacts already before the alkylation unit. With sufficient recycle of linear olefins, it is even possible to convert most of the linear olefins to isoolefins. There are several publications concerning the combination of etherification and skeletal isomerization.
However, in recent times, etherification has become questionable because of the water pollution caused by MTBE released from leaking gasoline storage tanks. There is a need for a process that would maximize the yield of useful fuel components from a light hydrocarbon feed without resorting to etherification.
An alternative to etherification of light isoolefins is to use them in dimerization, as disclosed in U.S. Pat. Nos. 3,325,465 and 6,613,108, DE Patent No. 3,542,171 and International Patent Application WO 01/46095.
A number of processes involving both dimerization and isomerization steps are also known in the art.
Thus, FR 2525171 discloses a combination of oligomerization and skeletal isomerisation of isobutene. The oligomerization is first performed and the oligomerate is separated. Then, butanes are separated by extractive distillation and the remaining butenes are skeletal-isomerized. The isomerized butene is circulated back to oligomerization. The process requires the use of extractive distillation for the separation of butanes. GB 595827 discloses a system with a sequence of oligomerization-isomerization-oligomerization and dehydrogenation for the production of high-grade motor or aviation fuels. RU 2165913 discloses a process for converting n-alkanes into isoalkenes. The essential components of the system are dehydrogenation and skeletal isomerization thereafter. Dehydrogenation is an expensive process and requires a costly investment in the process design and equipment. The process is carried out in the presence of polar compounds that later on need to be separated from the reaction product. This also is costly.
None of the above processes provides an integral process for producing from a fresh olefinic feedstock gasoline grade components, which meet present standards and requirements.